This invention generally relates to electrodes for use in electrochemical processes wherein it is desired to evolve oxygen at the anode and particularly where chloride ion is present in the electrolyte. Two prime examples of this are evident from the following discussion.
Several proposals have been suggested for sea-based power plants for deriving energy from ocean thermal gradients, wind and wave generators, and from nuclear breeder reactors placed at sea so as to minimize thermal pollution. A number of such proposals have suggested the direct electrolysis of seawater as a convenient source of hydrogen on a large scale. Such electrolytic hydrogen could then be shipped ashore or could be combined with carbon dioxide extracted from seawater to produce methane, methanol, and other light fuels for transportation to the land masses of the earth for use as an energy source. A major problem, however, exists in this area in that the usual electrode materials and conditions of electrolysis for seawater favor the evolution of chlorine anodically rather than oxygen and thus massive quantities of by-product chlorine would necessarily be generated by any such major power plant. Such generated by-product chlorine could not be discharged to the environment even at mid-ocean and would be extremely costly to convert back to chloride. By the practice of the instant invention, the chlorine evolution at the anode of such a system would be essentially eliminated and oxygen would instead be released at said anode, obviating all of the expensive methods required to convert chlorine gas back to a chloride form.
In various other electrochemical processes such as, for example, in the production of chlorine and other halogens, the production of chlorates, the electrolysis of other salts which undergo decomposition under electrolysis conditions, it has recently become commercially possible to use dimensionally stable electrodes in place of graphite or the like. These dimensionally stable electrodes usually have a film-forming valve metal base such as titanium, tantalum, zirconium, aluminum, niobium and tungsten, which has the capacity to conduct current in the cathodic direction and to resist the passage of current in the anodic direction and are sufficiently resistant to the electrolyte and conditions used within an electrolytic cell, for example, in the production of chlorine and caustic soda, to be used as electrodes at electrolytic processes. In the anodic direction, however, the resistance of the valve metals to the passage of current goes up rapidly, due to the formation of an oxide layer thereon, so that it is no longer possible to conduct current in the electrolyte in any substantial amount without substantial increase in voltage which makes continued use of uncoated valve metal electrodes in an electrolytic process uneconomical.
It is, therefore, customary to apply electrically conductive electrocatalytic coatings to these dimensionally stable valve metal electrode bases. The electrode coatings must have the capacity to continue to conduct current to the electrolyte over long periods of time without becoming passivated, and in chlorine production must have the capacity to catalyze the formation of chlorine molecules from the chloride ions at the anode. Most of the electrodes utilized today catalyze the formation of chlorine molecules. These electroconductive electrodes must have a coating that adheres firmly to the valve metal base over long periods of time under cell operating conditions.
The commercially available coatings contain a catalytic metal or oxide from the platinum group metals, i.e., platinum, palladium, iridium, ruthenium, rhodium, osmium, and a binding or protective agent such as titanium dioxide, tantalum pentoxide and other valve metal oxides in sufficient amount to protect the platinum group metal or oxide from being removed from the electrode in the electrolysis process and to bind the platinum group metal or oxide to the electrode base. Other such electrocatalytic coatings are described in U.S. Pat. No. 3,776,384, U.S. Pat. No. 3,855,092, U.S. Pat. No. 3,751,296, U.S. Pat. No. 3,632,498, and U.S. Pat. No. 3,917,518. Any of the foregoing electrodes, whether carbon, metallic, electrocatalytic coated valve metal, or the like, are useful in the practice of the instant invention as each may serve as the base for the oxygen-selective coating of the instant invention.
In anodes for the recovering of metals by electrowinning, a continual source of difficulty has been the selection of a suitable material for the anode. The requirements are insolubility, resistance to the mechanical and chemical effects of oxygen liberated on its surface, low oxygen overvoltage, and resistance to breakage in handling. Lead anodes containing 6 to 15 percent antimony have been used in most plants. Such anodes are attacked by chloride if present in the electrolyte. This is the case in Chuquicamata, Chile, where it is necessary to remove cupric chloride dissolved from the ore by passing the solution over reducing material so as to reduce the cupric to insoluble cuprous chloride. This adds to the expense of the process immensely whereas by the use of an oxygen selective anode, the cupric chloride in solution would not be evolved as chlorine gas to any great extent, and thus eliminating the need for the reduction of the cupric chloride to insoluble cuprous chloride.